(1) Field of the Invention
The present invention relates generally to a still picture imaging apparatus applicable to electronic still cameras, particularly such imaging apparatus of the type including an imaging device using charge coupled devices (CCDs) with an overflow drain (OFD), and more particularly the invention relates to an apparatus for performing the measurement of incident light quantity, exposure control, etc., of the imaging device.
(2) Prior Art
With conventional still picture imaging apparatus of the type employing an electronic imaging device, it has been well known that during the photographing operation the quantity of incident light to the imaging device is preliminarily measured by utilizing the imaging device itself without using any separate photometer device.
In this type of still picture imaging apparatus which utilizes its imaging device for light quantity measurement, the signal resulting from the light quantity measurement is utilized for performing automatic feedback control (so-called auto-iris control) of the aperture stop in such a manner that the proper exposure is ensured during the imaging operation performed immediately after the light quantity measurement. The past practice has been such that during the light quantity measuring operation the imaging device is brought into operation as during the imaging operation and also the same aperture stop control as in the case of a video camera for moving pictures is effected to make up the dynamic range thereby taking out a part of the resulting video output signal as a measured light quantity signal. However, this method is disadvantageous in that during the light quantity measuring operation preceding the photographing the imaging device is operated in the like manner as during the photographing to control the exposure and therefore a greater driving power is required. Another disadvantage is that the video output signal detected as a measured light quantity signal is an integrated sampled data in time so that there is a limitation to the operating speed of the auto-iris control and a considerable time is required until the aperture stop is stabilized in such cases as immediately following the closing of the power source and when the exposure condition is changed rapidly.
On the other hand, a method has already been proposed (Japanese Patent Application Laid-Open No. 2-108924; published on Apr. 20, 1990) in which the imaging device is used for the measurement of light quantity. However the method of obtaining information for exposure control from other than a video output signal is such that the imaging device comprises charge coupled devices (CCDs) with an overflow drain (OFD) and photo current flowing into the OFD is detected as a measured light quantity signal. In this method, the imaging device comprises for example a CCD imaging device having a vertical overflow drain structure. During the light quantity measuring operation, a condition is established where the driving for the imaging operation including the charge transfer operation, etc., of the imaging device is entirely stopped. That is, all the bias voltages for the imaging device are cut off. In this condition, in response to an image of an object formed on the photosensitive section of the imaging device, a photo current flowing into the overflow drain (the CCD substrate) from the photodiodes constituting the CCD photosensitive picture elements is measured. This is used as a measured light quantity signal which is utilized in the automatic exposure control for determining the desired amount of exposure at that time.
In accordance with this method, however, there still exists a problem that if the photo current flowing into the overflow drain during the time interval from the light quantity measuring operation to the imaging operation and during the imaging operation is varied, the opening of the aperture stop cannot maintain the state at the time of the light quantity measurement. Thus, the exposure control becomes unstable. There exists still another problem that where the light quantity measuring operation for the next imaging is started after the completion of the current imaging operation, even if the operation of the imaging device is cut off, the charges stored in the capacitance components of the various circuit portions connected to the imaging device are left. Therefore, the next light quantity measuring operation cannot be started until these charges have been discharged completely thus giving rise to a problem for the imaging in a continuous shooting mode.